1. Field of the Invention
The present invention relates to a device for automatic focusing control for a video camera system or the like.
2. Description of the Prior Art
Various automatic focusing mechanisms have been proposed for use with imaging systems such as video camera systems. One general automatic focusing mechanism is known as a TTL (Through The Lens) system in which a video signal generated by an imaging device such as a CCD (charge-coupled device) or the like is processed by a microcomputer to measure the sharpness of the image on the CCD, and a lens combined with the imaging device is moved on the basis of the measured sharpness until the image is sharply focused on the CCD.
There are two typical automatic focusing variations based on the TTL principle, depending on how optical information from the subject to be imaged is produced. The first automatic focusing scheme is of a passive nature in that information borne by light coming from the subject and entering a master lens is used directly as automatic focusing information. The second automatic focusing configuration is active in that a light signal is radiated from the video camera system to the subject and information borne by light reflected by the subject is used as automatic focusing information. The active and passive automatic focusing variations operate mostly on the triangulation principles.
Recently, however, a passive automatic focusing process which utilizes the detected contrast of an image for focusing the image has also been in wide use. The conventional passive automatic focusing process of this type will be described below with reference to FIGS. 1A through 1D and 2 of the accompanying drawings.
When the image of a subject 2 formed in a view-finder 1 of a video camera is focused and hence can clearly be seen by the user of the video camera, as shown in FIG. 1A, a video signal 3 produced by the imaging device of the video camera and passed through a filter has a high contrast and high frequency components, as shown in FIG. 1B. When the image of the subject 2 is defocused, as shown in FIG. 1C, the video signal 3 has a low contrast and low frequency components, as shown in FIG. 1D. The passive automatic focusing process is based on this principle. The video camera has a zoom lens assembly composed of a focusing lens, a zoom lens, and a master lens. For the detection of the contrast of the image, the focusing lens, indicated at 6 in FIG. 2, is moved a small distance by an automatic focusing (AF) motor 5 which is energized by an AF control circuit 8 that is supplied with a signal to reach a maximum contrast of the video signal 3 which represents the image of the subject 2 formed on a CCD 7.
The conventional automatic focusing control device is constructed as shown in FIG. 3. As shown in FIG. 3, a video camera system or the like includes an imaging device 17 including a lens assembly composed of a focusing lens 17a, a zoom lens, and a master lens, and a CCD 7. The imaging device 17 also includes an AF motor (shown in FIG. 2) which can be driven by a drive signal from a lens driver 25 for moving the focusing lens 17a over a small distance in a direction indicated by the arrow 17b. Instead of the focusing lens 17a, the master lens may be moved as with an inner focusing lens.
As shown in FIG. 4A, the CCD 7 has a field 9 which includes an image frame 10 from which image data can be produced. Light from a subject to be imaged is transmitted through the lens assembly and applied to the CCD 7. A video signal generated by the CCD 7 is supplied through an amplifier 18 to a video signal processor 19 which in turn supplies a processed video signal to a video recording circuit. The amplifier 18 separates a luminance signal (hereinafter referred to as a "Y signal") from the video signal. The Y signal is supplied to a contrast detector 20 which detects the contrast of an image. In the contrast detector 20, a Y signal generated by the image frame 10 is passed through a bandpass filter (BPF) 20a, and then amplified by an amplifier 20b. Then, the amplified Y signal is applied to a detector 20c which detects the contrast data from the image frame 10. The contrast data from the image frame 10 are supplied to an analog-to-digital (hereinafter referred to as an "A/D") converter 22, which converts the supplied contrast data into digital contrast data. The digital contrast data are then supplied through a gate 21 to an integrator 23 which integrates the data per field. The integrated data are supplied to a CPU (central processing unit) 24. The CPU 24 controls the gate 21 and the A/D converter 22.
The device which is constructed as shown in FIG. 3 operates as follows: The Y signal from one horizontal scanning line 11 (FIG. 4A) within the image frame 10 in the CCD field 9 is separated as shown in FIG. 4B, and differentiated as shown in FIG. 4C. The differentiated contrast signal is then folded over as shown in FIG. 4D. The absolute contrast value is produced by the detector 20c and converted into digital data by the A/D converter 22. The digital data from the A/D converter 22 are sampled at a sampling frequency of about 4.7 MHz as shown in FIG. 4E. The contrast values at respective sampling points are added per horizontal scanning line. Then, the added contrast values per horizontal scanning line are integrated in the vertical direction in the image frame 10 by the integrator 23, thereby detecting the maximum contrast value in the image frame 10. Based on the maximum contrast value thus detected, the CPU 24 actuates the lens driver 25 to move the focusing lens 17a in directions indicated by the arrow 17b. The device shown in FIG. 3 is of the full integration type in which the contrast data are fully integrated in both the horizontal and vertical directions in the field 9.
FIG. 5 of the accompanying drawings illustrates another automatic focusing control device which holds peak values in the vertical direction. The device shown in FIG. 5 differs from the device shown in FIG. 3 in that the Y signal produced by the BPF 20a and the amplifier 20b is supplied to a vertical peak hold circuit 26 through the gate 21 which is controlled by the CPU 24. The vertical peak hold circuit 26 holds peak contrast values in the vertical direction in the image frame 10. Then, a peak detector 27 detects only the highest one of the peak contrast values on the respective lines. The detected peak contrast value is then converted by the A/D converter 22 into digital data, which are then supplied to the CPU 24.
The automatic focusing control device of the full integration type shown in FIG. 3 is disadvantageous in that its dynamic range is too wide. Furthermore, as shown in FIGS. 4A through 4E, the contrast values in each horizontal line in the image frame 10 are integrated vertically in one field or frame. Therefore, contrast changes in the horizontal direction and also contrast changes in the vertical direction are integrated. Inasmuch as the contrast changes in the field during one vertical period (lV) are large, the maximum contrast value tends to vary greatly even when the video camera system wobbles slightly or the subject moves slightly. Moreover, when an image 9a of high contrast as viewed by the user and an image 9b of low contrast as viewed by the user are formed in the field 9 as shown in FIGS. 6A and 6D, respectively, Y signals are produced from these images 9a, 9b as shown in FIGS. 6B and 6E, respectively. These Y signals are differentiated into contrast data as shown in FIGS. 6C and 6F, respectively. These detected contrast data are of the same average value, and hence cannot be distinguished.
The circuit arrangement shown in FIG. 5 holds only one maximum contrast value during one vertical period. Therefore, if the contrast data contain a noise N as shown in FIG. 7A, then the noise N is held as shown in FIG. 7B. Consequently, the device shown in FIG. 5 is vulnerable to noise. In addition, in the event that the image frame 10 in the field 9 contains images of a highly bright subject 9c, a dark area 9d, and a relatively dark area 9e, as shown in FIG. 7C, the maximum contrast level 9c' shown in FIG. 7D is held while the other lower contrast levels 9e', 9d' are being ignored. Accordingly, it is difficult to establish a gain for ordinary subjects if they contain highly bright areas or spots.
Generally, since the dynamic range of a Y signal produced by a video camera is wide, if the gain of the contrast detector is set for higher brightness, then the contrast of ordinary subjects of lower brightness cannot be properly detected, and if the gain of the peak detector is set for ordinary subjects of lower brightness, then the contrast of areas of higher brightness cannot be held at maximum level. Use of automatic gain control to avoid the above problem results in a complex contrast detector.